Catching an L means catching a loss. In this sense, you catch an L if you say something stupid or, if you try to sound intelligent but are proved wrong.

1. Sarah says, "Has anyone ever noticed that "Santa" has the same letters as Satan? Isn't that weird? What are your thoughts?"
Cathy responds, "And Santa is also the word for Saint in many languages. And since the figure of Santa Claus is based off SAINT Nicholas, its not surprising. So nice try, but you failed."

In that case, Sarah caught an L because she tried to prove an invalid point.

The Urban Dictionary T-Shirt

An L is a marijuana joint, created with 2 papers that before rolled, has an L shape to it, but normally the L is on its side with the long side going across instead of going up and down, and the smaller end going up and down, then you put the marijuana (preferably mixed with tobacco) in the L, roll it, then enjoy your L joint.
Contrary to popular believe, it's not technically a blunt, a blunt is a cigar re-rolled with marijuana, or you can just buy an already non-rolled blunt paper, an L is a good stepping stone to a blunt, you can fit 1g-1.5g in an L, whereas a blunt can go 3g-5g.

The Urban Dictionary T-Shirt

A cigar that is unwrapped/unraveled and refilled with marijuana and then smoked. Another name for this procedure is Blunt. This name was aquired by the brand Philly Blunts, which is a brand of cigar that is popular for this use. Other cigar brands that are popular for this use are Dutch Masters, El producto's, Sweedish Sweets and Garcia Vagas.

The Urban Dictionary T-Shirt

L Lawliet, commonly referred to by his alias "L", is a fictional character in the manga and anime series Death Note. He is considered the world's greatest detective, and he takes on the Kira investigation in the first arc.